1. Field of the Invention
This invention relates generally to closures, and more specifically to an improved closure for protecting internal threads of different types such as, for example, Buttress and American Petroleum Institute (API) threads at the end of a pipe, pipe coupling or the like.
2. Description of the Prior Art
It is known in the prior art to provide plastic closures for protecting the internal threads on the end of a pipe or pipe coupling secured to a pipe. Some of such closures are not threaded and are merely pressed or hammered into the openings in the couplings. One disadvantage of such closures is that each closure is designed for a particular type of thread and taper, thereby necessitating a large inventory of different but substantially indistinguishable closures. Another disadvantage is that expansion and contraction differences between the plastic and metal cause the closures to fall off or be knocked out of the pipe couplings since no means are provided other than friction to hold the closures in place. Other closures are provided with inner peripheral threads by which the closures may be screwed into threaded engagement with mating internal complimentary threads in the pipe coupling. Once again, due to the large number of different tapers and threads for any one size diameter pipe or pipe coupling, a large number of closures of varied design are required. For example, a closure for a pipe coupling of one size having an internal Buttress type thread in which the threads are of one diameter (at one distance from the coupling end) and extend to the end of the pipe cannot be used on the same size pipe coupling having an internal API thread in which the threads are of a smaller diameter (at the same distance from the coupling end) and do not extend to the end of the coupling.
It is further known to provide metal closures having outer peripheral threads for engaging the internal Buttress and API type threads. In addition to some of the disadvantages of the prior art plastic closures, such metal closures suffer from the added disadvantage that upon threading the closure into the end of the pipe or pipe coupling, cross threading may occur. Such cross threading may damage the internal threads necessitating retapping or cutting off the damaged end and re-threading it. Another disadvantage of the metal closures is that rusting and corrosion may occur at the interface of the closure and pipe coupling or the parts will freeze together in winter necessitating the services of a welder to remove the rusted on or frozen closure.